1. Field of the Invention
The present invention relates to a novel method of preventing undesired binding to the solid phase in solid phase assays, and especially those based upon mass-measuring techniques such as surface plasmon resonance.
2. Description of Related Art
Undesired non-specific or specific binding from serum or plasma is a common problem in immunological tests and has been described in the literature, especially with regard to label dependent tests, such as RIA, ELISA or FIA. Such undesired binding may cause several problems. For example, in the case of a sandwich type of assay where the labelled antibody binds non-specifically to the solid support, this binding will decrease the signal to noise ratio hampering the performance of the test. Similarly, in assays for detection of specific antibodies corresponding only to a minor fraction of the immunoglobulin contents of the sample, non-specific binding of immunoglobulins contained in the sample to the solid support will negatively affect the performance of the test.
Various means to reduce these types of non-specific binding have been attempted. For instance, treatment of the immobilized capturing antibody surface has been done with serum proteins and non-ionic detergents, as disclosed in e.g. EP-A-0484765 and U.S. Pat. No. 4,829,009, with carboxy containing polymers as disclosed in e.g. EP-A-0372413, as well as with non-ionic block copolymers, as disclosed in e.g. EP-A-0468481. All these examples focus to prevent the non-specific binding of either the immunoreagents used for assaying the analyte or the analyte in the sample.
It is also known to reduce undesired binding by adding an "irrelevant" or non-specific antibody to the sample, i.e. an antibody that is not directed to the immobilized capturing antibody or to the analyte of interest, such irrelevant antibody capturing interfering plasma or serum components thereby at least partially inhibiting undesired binding of such components to the solid phase.
It is readily understood that the problems of undesired binding are especially pronounced in immunoassays based on mass-measuring techniques, i.e. where the mass of adsorbed or bound analyte and/or secondary reagent is measured rather than the intensity of the labels of specifically bound labelled components. One type of such mass-measuring technique is based on evanescent wave sensing, such as methods based on surface plasmon resonance, hereinafter for brevity SPR.
In an SPR based biosensor, changes in the refractive index in a layer close to a thin metal film are detected by the consequential changes of the intensity of a totally reflected light beam, and more particularly by the observed wavelength shifts for the intensity minimum. For a more detailed description of such a biosensor reference can be made to WO 90/05295 relating to an optical biosensor system, and to WO 90/05305 relating to a sensor unit and its use in biosensor systems.
One suitable type of sensing surface for use in SPR biosensors is described in WO 90/05303 and comprises a film of a free electron metal, preferably silver or gold, having one of its faces coated with a densely packed monolayer of specific organic molecules. To this monolayer a biocompatible porous matrix, e.g. a hydrogel, is bound, which matrix is employed for immobilizing a suitable ligand for a target biomolecule to be determined by the particular biosensor. Such a hydrogel may e.g. be based on dextran or a dextran derivative.
In immunoassays of serum or plasma samples with this type of hydrogel-coated sensor having specific antibody ligands immobilized thereto the problem of undesired binding resides in binding of non-analyte components in the sample to the solid phase surface material supporting the ligands. Such undesired binding not only causes a reduced sensitivity of the assay but also causes other problems, which can affect the possibility of regenerating the sensing surface for multiple analysis purposes, i.e. removing bound analyte from antibodies immobilized to the sensing surface to prepare the surface for a new analytical cycle. Further, the background levels will vary with different serum and plasma samples.
The prior art means discussed above for preventing non-specific binding of the analyte or the immunoreagents used for assaying the analyte do not solve these problems. Neither is the addition of irrelevant antibody to the sample as described above sufficient to reduce the problem of undesired binding to the sensing surface to an acceptable level.